It is standard to drive a dual-worm extruder by a transmission system having at an inner or input side a single input shaft and at an opposite outer or output side a pair of output shafts that can rotate co- or counterdirectionally. Typically the input shaft is directly connected to the output shaft of an electric motor and the output shafts are connected directly to the respective worms of the extruder. Thus the drive includes, in addition to the input shaft, motor, and output shafts, a transmission that steps down the motors rotation to increase torque, and one that splits the motor's force between the two output shafts.
In German Pat. document No. 2,801,138 filed Jan. 12, 1978 by H. Muenster the input shaft is journaled in the drive housing and carries the sole planet gear of a planetary-gear transmission including a sun gear carrying one of the output shafts and a ring gear rotatable in the housing. A gear identical to the driven planet gear meshes only with the ring gear and is carried on the other output shaft which is relatively close to the output shaft of the sun gear. This divides the torque of the input shaft between the two output shafts and rotates them oppositely. The inner ends of the two output shafts are supported in appropriate axial-thrust bearings in the drive housing, as these shafts normally are subjected to considerable axial reaction forces from the plastic mass being extruded.
German Pat. document No. 2,619,019 filed Apr. 30, 1976 by H. Selbach et al and the publication "esde aktuell" describe another system using a freely rotating ring gear and a row of two small and one large gear internally bridging it diametrally. The central small gear is carried on the input shaft, carries one of the output shafts, and meshes only with the large gear and the other small gear. The large gear and the other small gear also mesh with the ring gear, and the other small gear carries the other output shaft.
In both these arrangements the motor force is split and follows offcenter routes to the two output shafts. In addition the ring gear is loaded assymetrically, greatly decreasing its service life. Furthermore, it is normally necessary to provide a very long ring gear, one frequently twice as long as any other gear in the drive.
Splitting the load in these drives directly at the ring gear entails the use of at least three different gear engagements. Such operation requires that this structure be built to very close tolerances, supported accurately, and maintained meticulously to give good service. This problem is particularly aggravated in modern-day extruders which have very small-diameter worms that operate against great back pressure effective both axially and angularly on the drive.